Abstract

We consider the use of single-photon counting detectors in coherence-domain imaging. Detectors operated in this mode exhibit reduced noise, which leads to increased sensitivity for weak light sources and weakly reflecting samples. In particular, we experimentally demonstrate the possibility of using superconducting single-photon detectors (SSPDs) for optical coherence-domain reflectometry (OCDR). These detectors are sensitive over the full spectral range that is useful for carrying out such imaging in biological samples. With counting rates as high as 100 MHz, SSPDs also offer a high rate of data acquisition if the light flux is sufficient.

Photon-counting OCDR experimental arrangement using a Michelson interferometer comprising a beam-splitter (BS) and two mirrors. Mirror 1 is translated to change the length of the reference arm. Collinear spontaneous parametric downconversion generated in a 1.5-mm-thick BBO nonlinear-optical crystal (NLC), cut for type-I phase matching, serves as the optical source. D1 and D2 are dichroic components that direct the 532-nm output of the doubled Nd:YVO4 pump laser to the NLC. Dichroic D3 and Glan–Taylor polarizers P1 and P2 are used to remove unwanted wavelengths. Experiments were performed using both SPADs and SSPDs as photon-counting detectors.

OCDR interferograms measured with SPAD and SSPD single-photon detectors using the apparatus depicted in Fig. 6. A reduction in the full-width at half maximum (FWHM), corresponding to an improvement in axial resolution, is observed with the SSPD. This is a result of its broader spectral sensitivity.

Fourier transforms of the interference signals shown in Fig. 7, plotted as a function of wavelength. It is evident that the SPAD is not sensitive to wavelengths beyond 1100 nm, whereas the SSPD is sensitive in this region.

(a). Mean count rate observed at different positions of the reference mirror. Error bars denote ±1 standard deviation. Mean and standard deviations were estimated by taking 100 samples at each mirror position. (b). Ratio of count variance to count mean at different positions of the reference mirror. The measured value of this ratio is close to unity at all positions.

Single-photon axial scan of a 90-µm-thick silica window obtained with a scanning speed of 1 mm/sec and a counting time of 10 µsec per data point. The distance between the peaks is 134 µm, corresponding to the optical pathlength.